The present invention relates polymerization initiator compositions for polymerizing ethylenically unsaturated compounds and to photopolymerization initiator compositions having high sensitivity which are sensitive to a small amount of exposure from ultraviolet rays and visible rays and which can be used in connection with color proofing films.
Certain organic compounds are known to dissociate in the presence of light and generate free radicals. Some of the more common photolyzable organic compounds generate free halogen, which is a good hydrogen abstractor and will form hydrogen halide in the presence of hydrogen donors. The generation of free radicals in photosensitive systems containing photolyzable compounds has found many practical uses, particularly in the graphic arts field. Such systems have been described for free radical photography (e.g. print-out and bleach systems), free radical photopolymerization and photoinduced acid catalyzed reactions of many types, many of which have found valuable application in printing, duplicating, copying and other imaging systems.
It is well known that monomers, dimers, trimers, oligomers and prepolymers which have unsaturated bonds in the molecule can be polymerized in a presence of a photo polymerization initiator. Further, this phenomenon has been widely utilized in photo polymerization of photoresists printing plates, and color proofing films.
Useful known photo polymerization initiators include various substances which have been found to be practical. For example, benzoin, benzoin methyl ether, or benzoin ethyl ether, etc., carbonyl compounds such as benzil, benzophenone, acetophenone, Michler's ketone, azo compounds such as azobisisobutyronitrile or azodibenzoyl, etc., sulfur compounds such as dibenzothiazolyl sulfide or tetraethylthiuram disulfide, etc., halogen compounds such as carbon tetrabromide or tribromophenyl sulfone, etc., and 1,2-benzanthraquinone, etc.
However, these initiators are not sufficiently satisfactory. When they are used for polymerization of various ethylenically unsaturated compounds, they do not always have good sensitivity as photopolymers.
Sensitizers have been employed to broaden the spectral response of photosensitive compounds, especially in the visible portion of the light spectrum.
Furthermore, it is desired to have high sensitivity photoinitiators which can be employed in compositions which can be exposed by laser light or by projection through a mask. Typical initiators have a low quantum efficiency and therefore require the use of a high powered u.v. light source while an exposure mask is held in close contact with an exposure mask in a vacuum frame. In projection exposure, light, preferably visible light, is projected through a mask and a lens onto a light sensitive target film. To allow exposure, initiation sensitivity must be quite high and must be of an effective wavelength.
As described above, the prior photo-polymers are not satisfactory with respect to both sensitivity and their sensitive wavelength range, and they are, particularly, unsuitable as light-sensitive materials for visible light laser.
Still further, in the field of color proofing films, the color condition of the image formed is critical. Color proofing films typically comprise a transparent substrate and a colored photosensitive composition on the substrate. Color matching to an original image is crucial and therefore it is important that the non-colorant ingredients of the photosensitive composition contribute as little color modification as possible. The initiator composition of this invention permits the use of a lower amount of the initiator ingredient in the colored photosensitive composition and hence a lower amount of color contamination.